Note [Defaulting during simplifyInfer]

When we are inferring a type, we simplify the constraint, and then use
approximateWC to produce a list of candidate constraints.  Then we MUST

  a) Promote any meta-tyvars that have been floated out by
     approximateWC, to restore invariant (WantedInv) described in
     Note [TcLevel invariants] in GHC.Tc.Utils.TcType.

  b) Default the kind of any meta-tyvars that are not mentioned in
     in the environment.

To see (b), suppose the constraint is (C ((a :: OpenKind) -> Int)), and we
have an instance (C ((x:*) -> Int)).  The instance doesn't match -- but it
should!  If we don't solve the constraint, we'll stupidly quantify over
(C (a->Int)) and, worse, in doing so skolemiseQuantifiedTyVar will quantify over
(b:*) instead of (a:OpenKind), which can lead to disaster; see #7332.
#7641 is a simpler example.